2,473 research outputs found
Potential-energy (BCS) to kinetic-energy (BEC)-driven pairing in the attractive Hubbard model
The BCS-BEC crossover within the two-dimensional attractive Hubbard model is
studied by using the Cellular Dynamical Mean-Field Theory both in the normal
and superconducting ground states. Short-range spatial correlations
incorporated in this theory remove the normal-state quasiparticle peak and the
first-order transition found in the Dynamical Mean-Field Theory, rendering the
normal state crossover smooth. For smaller than the bandwidth, pairing is
driven by the potential energy, while in the opposite case it is driven by the
kinetic energy, resembling a recent optical conductivity experiment in
cuprates. Phase coherence leads to the appearance of a collective Bogoliubov
mode in the density-density correlation function and to the sharpening of the
spectral function.Comment: 5 pages, 4 figure
Shot Noise Probing of Magnetic Ordering in Zigzag Graphene Nanoribbons
The nonequilibrium time-dependent fluctuations of charge current have
recently emerged as a sensitive experimental tool to probe ballistic transport
through evanescent wave functions introduced into clean wide and short graphene
strips by the attached metallic electrodes. We demonstrate that such
"pseudo-diffusive" shot noise can be substantially modified in zigzag graphene
nanoribbon (ZGNR) due to the topology of its edges responsible for localized
states that facilitate ferromagnetic ordering along the edge when Coulomb
interaction is taken into account. Thus, the shot noise enhancement of
unpolarized, and even more sensitively of spin-polarized, charge currents
injected into ZGNR will act as an all-electrical and edge-sensitive probe of
such low-dimensional magnetism.Comment: 5 pages, 3 color figures; references update
Optimal T of cuprates: role of screening and reservoir layers
We explore the role of charge reservoir layers (CRLs) on the superconducting
transition temperature of cuprate superconductors. Specifically, we study the
effect of CRLs with efficient short distance dielectric screening coupled
capacitively to copper oxide metallic layers. We argue that dielectric
screening at short distances and at frequencies of the order of the
superconducting gap, but small compared to the Fermi energy can significantly
enhance T, the transition temperature of an unconventional superconductor.
We discuss the relevance of our qualitative arguments to a broader class of
unconventional superconductors.Comment: 8 Pages, 4 figure
Emergence of gravity from interacting simplices
We consider a statistical model of interacting 4-simplices fluctuating in an
N-dimensional target space. We argue that a gravitational theory may arise as a
low energy effective theory in a strongly interacting phase where the simplices
form clusters with an emergent space and time with the Euclidean signature. In
the large N limit, two possible phases are discussed, that is, `gravitational
Coulomb phase' and `gravitational Higgs phase'.Comment: 18 pages, 6 figures, one-column format; major revisions in version 5
: reviews on emergent gauge theories added; microscopic simplex model for
emergent gravity added; erroneous statements on diffeomorphism invariance
remove
Cyclic exchange, isolated states and spinon deconfinement in an XXZ Heisenberg model on the checkerboard lattice
The antiferromagnetic Ising model on a checkerboard lattice has an ice-like
ground state manifold with extensive degeneracy. and, to leading order in J_xy,
deconfined spinon excitations. We explore the role of cyclic exchange arising
at order J^2_xy/J_z on the ice states and their associated spinon excitations.
By mapping the original problem onto an equivalent quantum six--vertex model,
we identify three different phases as a function of the chemical potential for
flippable plaquettes - a phase with long range Neel order and confined spinon
excitations, a non-magnetic state of resonating square plaquettes, and a
quasi-collinear phase with gapped but deconfined spinon excitations. The
relevance of the results to the square--lattice quantum dimer model is also
discussed.Comment: 4 pages, 5 figure
The electronic structure and the phases of BaVS3
BaVS3 is a moderately correlated d-electron system with a rich phase diagram.
To construct the corresponding minimal electronic model, one has to decide
which d-states are occupied, and to which extent. The ARPES experiment
presented here shows that the behavior of BaVS3 is governed by the coexistence
of wide-band (A_1g) and narrow-band (twofold degenerate E) d-electrons. We
sketch a lattice fermion model which may serve as a minimal model of BaVS3.
This serves foremost for the understanding of the metal-insulator in pure BaVS3
and its absence in some related compounds. The nature of the low temperature
magnetic order differs for several systems which may be described in terms of
the same electron model. We describe several recent experiments which give
information about magnetic order at high pressures. In particular, we discuss
field-induced insulator-to-metal transition at slightly subcritical pressures,
and an evidence for magnetic order in the high-pressure metallic phase. The
phase diagram of Sr-doped BaVS3 is also discussed. The complexity of the phases
of BaVS3 arises from the fact that it is simultaneously unstable against
several kinds of instabilities.Comment: Presented at the International Conference on Magnetism 2006 (Kyoto),
6 pages, 9 figure
k-Spectra of weakly-c-Balanced Words
A word is a scattered factor of if can be obtained from by
deleting some of its letters. That is, there exist the (potentially empty)
words , and such that and
. We consider the set of length- scattered
factors of a given word w, called here -spectrum and denoted
\ScatFact_k(w). We prove a series of properties of the sets \ScatFact_k(w)
for binary strictly balanced and, respectively, -balanced words , i.e.,
words over a two-letter alphabet where the number of occurrences of each letter
is the same, or, respectively, one letter has -more occurrences than the
other. In particular, we consider the question which cardinalities n=
|\ScatFact_k(w)| are obtainable, for a positive integer , when is
either a strictly balanced binary word of length , or a -balanced binary
word of length . We also consider the problem of reconstructing words
from their -spectra
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